Photoinduced switching liquid crystal device utilizing photoisomerization

ABSTRACT

A fast and highly practical photoinduced switching liquid crystal device is provided. 
     In the photoinduced switching liquid crystal device, in-plane switching of an optical anisotropic axis of dichroic nematic liquid crystal is performed at a high speed by light.

TECHNICAL FIELD

The present invention relates to photoinduced switching liquid crystaldevices.

BACKGROUND ART

Relevant art of the present invention is disclosed in the followingliteratures.

(1) A. S. Zolotko, V. F. Kitaeva, N. Kroo, N. N. Sobolev, L. Csillag,JETP.Lett., 32, 158 (1980).

(2) B. Y. Zel'dovich, N. V. Tabiryan, Sov. J. Quantum Electron., 10, 440(1980).

(3) T. Ikeda, T. Sasaki, K. Ichimura, Nature, 361, 428 (1993).

(4) L. Komitov, K. Ichimura, A. Strigazzi, Liq. Cryst., 27, 51 (2000).

(5) L. Komitov, C. Ruslim, Y. Matsuzawa, K. Ichimura, Liq. Cryst., 27,1011 (2000).

(6) L. Komitov, K. Ichimura, Mol. Cryst. Liq. Cryst., in press

(7) L. Komitov, O. Tsusumi, C. Ruslim, T. Ikeda, K. Ichimura, K.Yoshino, submitted to J. Appl. Phys.

(8) E. Santamato, Y. R. Shen, “Liquid Crystals for Nonlinear OpticalStudies” in the book “Handbook of Liquid Crystal Research” OxfordUniversity Press, New York, 1997.

(9) C. Ruslim, L. Komitov, Y. Matsuzawa, K. Ichimura, Jap. J. Appl.Phys., 39, L104 (2000).

(10) L. Komitov, J. Yamamoto, H. Yokoyama, J. Appl. Phys., 89, 7730(2001).

(11) P. Jägemalm, G. Barbero, L. Komitov, A. K. Zvezdin, Phys. Rev. E,58, 5982 (1998).

(12) P. Jägemalm, G. Barbero, L. Komitov, A. Strigazzi, Phys. Lett.A235, 621 (1997).

(13) M. Monkade, M. Boix, G. Durand, Europhys. Lett., 5, 697 (1988).

(14) B. Jerome, M. Boix, P. Pieranski, Europhys. Lett., 5, 693 (1988).

(15) P. Jägemalm, L. Komitov, Liq. Cryst., 23, 1 (1997).

(16) M. Nobili, PhD Thesis, 1992.

(17) M. Nobili, G. Durand, Europhys, Lett., 25, 527 (1994).

(18) P. Jägemalm, D. S. Hermann, L. Komitov, F. Simoni, Liq. Cryst, 24,335 (1998).

(19) D. S. Hermann, P. Rudquist, K. Ichimura, K. Kudo, L. Komitov, S. T.Lagerwall, Phys. Rev. E, 55, 2857 (1997).

-   -   (20) Y. Matsuzawa, C. Ruslim. L. Komitov, K. Ichimura, Mol.        Cryst. Liq. Cryst. in press.

Liquid crystal is a very highly anisotropic material and has opticalproperties which can be very easily changed in accordance with variousexternal factors such as electric and magnetic fields, mechanical flow,temperature, and light. In addition to the electro-optic effects,optically induced reorientation of the alignment of liquid crystal hasdrawn strong interest due to its potential for different deviceimplementations in photonics. In general, there are two possible methodsfor affecting the alignment of liquid crystal by light so as to changelight transmittance thereof. One of such methods is a way to use directinteraction between light and liquid crystal molecules, as is thephotoexcited Fredericks transition, and the other method is an indirectphotoexcitation method in which surface or bulk liquid crystalproperties are changed by light.

The photoexcited Fredericks transition is caused by giant opticalnonlinearity of liquid crystal and has drawn strong interest for these20 years. In this case, when light imposes a direct and rotationaltorque on liquid crystal molecules, reorientation of the alignmentthereof occurs in a predetermined direction. The direction in which theliquid crystal is aligned depends on various experimental conditionssuch as an alignment direction of liquid crystal before illumination,the thickness of a cell, and light intensity.

In recent years, photoinduced reorientation of the alignment ofazobenzene liquid crystal was reported which is caused by the change inbulk or surface properties thereof resulting from photoexcitation.

A subject which is most closely related to the study carried out by theinventors of the present invention is a photoinduced anchoringtransition of dichroic azobenzene liquid crystal that occurs resultingfrom the modulation of macroscopic surface anchoring conditions inducedby the change in molecular shape during a photoisomerization process.That is, when the concentration of the azobenzene molecules adheres tothe solid surface exceeds a certain level, the macroscopic alignment ofliquid crystal is changed from the planar alignment to the homeotropicalignment. That is, as reported by the inventors of the presentinvention, after being transformed from trans-isomers to cis-isomers byphotoisomerization, the azobenzene molecules are more likely to adhereselectively to a solid surface, and as a result, the anchoringconditions for the liquid crystal are changed. The reason for this isunderstood that since the molecular shape and the direction of molecularelectron dipole moment of the trans-isomer are significantly differentfrom those of the cis-isomer, a large polarity resulting from a bentmolecular shape of the cis-isomer enhances the adsorption properties ofthe azobenzene molecules to the solid surface.

However, the modulation (photoregulation of the anchoring) of thesurface anchoring conditions caused by photoexcitation is a continuousprocess, and hence it is not appropriate that this phenomenon itself isused as the principle of a light switching device. In fact, even beforethe transformation from the planar alignment to the homeotropicalignment caused by photoexcitation is observed, the continuous changein anchoring intensity, which depends on the light exposure time, can beclearly grasped by the change in voltage when the threshold voltage ofthe electric field induced Fredericks transformation is measured.

DISCLOSURE OF INVENTION

Recently, the inventors of the present invention reported the couplingbetween anchoring interaction in a azimuthal angle direction and that ina polar angle direction, which is shown when the direction of alignmentof liquid crystal with respect to a substrate is represented by aazimuthal angle φ and a polar angle θ. In particular, in the case of thealignment of nematic liquid crystal by a SiO_(x) thin film, it was foundthat biaxial degeneracy of the alignment occurs. That is, due to thepresence of the coupling of two anchoring forces, quasi-in-plane(in-plane) switching with respect to the change in direction of thealignment of liquid crystal can be performed by the balance between theazimuth anchoring and the polar anchoring.

According to the present invention, a twofold degenerate (two-statedegeneracy) in-plane switching phenomenon, which is caused by thecompetition of the anchoring forces on the SiO_(x) substrate, iscombined with an anchoring transition phenomenon, which is caused byconventional photoisomerization of azobenzene liquid crystal molecules,thereby realizing photoregulated fast in-plane switching. According tothe present invention, by photoexcitation, the angle of an anisotropicaxis can be rotated by approximately 90°, the switching time can be onesecond or less, and as a result, a significantly fast switching can berealized as compared to that performed by a conventional photoexcitedin-plane switching control method.

Accordingly, in consideration of the situation described above, anobject of the present invention is to provide a fast and highlypractical photoinduced switching liquid crystal device.

To the end described above, the present invention provides:

[1] a photoinduced switching liquid crystal device which ischaracterized in that fast in-plane photoinduced switching of an opticalanisotropic axis of dichroic nematic liquid crystal is performed bylight.

[2] In the photoinduced switching liquid crystal device according to theabove [1], ultraviolet light is used as the light described above sothat azobenzene liquid crystal molecules are transformed fromtrans-isomers to cis-isomers.

[3] In the photoinduced switching liquid crystal device according to theabove [1], the structure is formed in which the liquid crystal describedabove is enclosed in a cell formed of substrates provided with obliquelydeposited silicon oxide (SiO_(x)) functioning as an alignment layer, andthe switching is performed based on the photoinduced anchoringtransition.

[4] In the photoinduced switching liquid crystal device according to theabove [1], [2], or [3], the change in the optical anisotropic axiscaused by photoinduction can be increased to approximately 80 to 90°.

[5] In the photoinduced switching liquid crystal device according to theabove [1], [2], or [3], the switching time described above is fast onthe second time scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an experimental device for a photoinducedswitching liquid crystal device of an example according to the presentinvention.

FIG. 2 is a view schematically showing the region of a twofolddegenerate anchoring according to the present invention.

FIG. 3 is a view showing the temperature dependence of an azimuthalangle φ according to the present invention.

FIG. 4 includes views showing photographs in which a sample illuminatedwith light according to the present invention was observed at twodifferent positions by the use of crossed nicols.

FIG. 5 is a schematic view of trans-cis photoisomerization of a4-hexyloxy-(4′-hexyl) azobenzene molecule according to the presentinvention.

FIG. 6 is a schematic view showing the state in which a cis-isomeraccording to the present invention is fixed to a solid surface.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail.

First, photoinduced in-plane (in-plane) switching of photodichroicnematic liquid crystal will be described.

In this embodiment, fast in-plane switching of photodichroic nematicliquid crystal by photoexcitation will be described.

In the alignment control of nematic liquid crystal by a obliquelydeposited SiO_(x) layer, degenerated biaxial anchoring is achieved. Theprocess of this in-plane switching occurs by competition between twophenomena, that is, the photoisomerization which modulates the anchoringconditions and the coupling of anchoring forces present in a polar angledirection and in a azimuthal angle direction. In the photoinducedin-plane reorientation of the alignment, a rotation of an anisotropicaxis of approximately 90° was found. Together with the model of thephotoinduced in-plane switching, some of new effects will be brieflydescribed.

In the present invention, a general sandwich type liquid crystal cellwas used. This cell was composed of two glass substrates each having anITO electrode coated with an obliquely deposited SiO_(x) thin film,thereby obtaining twofold degeneracy of the alignment of nematic liquidcrystal. The deposition conditions of a SiO_(x) thin film aresignificantly important for realizing the twofold degenerate anchoringcondition described above [for more detailed information, referabove-mentioned literatures (13) to (15)]. The cell gap was set to 6 μm(obtained by SiO₂ spacers which were deposited by a general method), andthe liquid crystal material was introduced into the cell in theisotropic phase.

The dichroic nematic liquid crystal used in this embodiment was4-hexyloxy-(4′-hexyl)azobenzene having the following molecular structure(FIG. 5).

Due to the twofold degenerate anchoring condition, four different typesof domain structures could be found, that is, two twisted structureshaving different senses from each other and two uniformly tiltedstructures in which the tilt directions of molecules are presentsymmetrically on both sides of the deposited SiO_(x) plane. However,when special treatment is performed, a sample having a monodomainstructure can be prepared which is one of the uniformly tiltedstructures. The photoexcited reorientation of the alignment of theliquid crystal was measured using an experimental device describedbelow.

FIG. 1 is a schematic view of an experimental device for a photoinducedswitching liquid crystal device of an example according to the presentinvention.

In this figure, as an excitation light source, an Hg lamp 5 is used. Thewavelength of excitation light is selected by using a optical filter 6.By illumination of the excitation light, photoisomerization occurs indichroic nematic liquid crystal. Light being emitted from a white lightsource of a halogen lamp 1 and passing through an optical filter 2,which has a wavelength λ=nm (λ>580 nm), is used for observation of thealignment of the liquid crystal.

This experimental device described above is formed of afluorescent/polarizing microscope (Nikon, Eclipse800), a highly advancedCCD (charge coupled device) camera 9 connected to a computer 8, asoftware for image processing, a hot stage 4 for temperature control,and two light sources 1 and 5 for the observation of the liquid crystaland for the photoisomerization, respectively. The sample is placed inthe hot stage 4 for temperature control, which is mounted on a rotationstage of the polarizing microscope. The alignment of the liquid crystalin the cell was inspected between polarizers orthogonally intersectingwith each other by using light passing through the optical filter 2which cut a wavelength of 580 nm or less. In addition, reference numeral7 indicates a dichroic mirror.

The cut-off wavelength of the filter 2 used for the light source forobservation was selected so as to avoid the photoisomerization ofdichromatic nematic liquid crystal 3 during observation. The lightsource needed for promoting the photoisomerization process was generatedby the Hg lamp 5. The excitation of the sample was performed by awavelength λ of 510 to 560 nm. The alignment direction of the liquidcrystal caused by photoorientaion, that is, the anisotropic axis of thesample, was measured as the position at which the transmitting lightbetween the two polarizers orthogonally intersecting each other was theminimum.

FIG. 2 is a view schematically showing the region of the twofolddegenerate anchoring according to the present invention.

This figure is a schematic view of nematic liquid crystal in the twofolddegenerate anchoring condition which is induced by the obliquelydeposited SiO_(x) at an angle α (in order to avoid complication, onlyone of two possible alignment directions of the liquid crystal is shownin the figure. The other one is located symmetrically on the other sidewith respect to the deposition plane YZ). The alignment direction of theliquid crystal, that is, the direction of the optical anisotropic axis,is determined by φ and θ, that is, the azimuthal angle and the polarangle, respectively.

FIG. 2( a) shows the temperature-induced change of the opticalanisotropic axis, in which positions 1 and 2 indicate the alignmentdirections of the liquid crystal at temperatures T₁ and T₂,respectively, and T₁<T₂ is satisfied. Along the trajectory on a planedefined by equations (1) and (2), which is perpendicular to the YZ planeand which forms an angel γ with an XY plane, the alignment direction ofthe liquid crystal moves away from the deposition plane YZ. FIG. 2( b)shows the photoinduced change of the optical anisotropic axis, and as isthe case shown in FIG. 2( a), the optical axis of the sample moves fromthe deposition plane to the X axis. (For more detailed information,refer above-mentioned literatures (15) to (17).)

The region of the twofold degenerate anchoring is defined between twodifferent monostable alignment states, that is, planar and tiltedalignment states. The directions of the planar alignment and the tiltedalignment of the liquid crystal are along the X axis direction and onthe YZ plane, respectively. In the region of the twofold degeneracy, thealignment direction of the liquid crystal splits into two directionsthat are symmetrically located on both sides of the deposition plane. Asit has been theoretically predicted and experimentally confirmed, thereshould exist a coupling between anchoring in the polar angle directionand that in the azimuthal angle direction, each angles being defined byθ and φ, respectively. In the case of the temperature-induced alignmenttransition, the coupling is represented by the following equation.

$\begin{matrix}{{\sin\;{\varphi tan\theta}} = \sqrt{\frac{1 - \left( {r + {t/S} + \rho + {\tau/S}} \right)}{r + {t/S}}}} & (1)\end{matrix}$cos φ tanθ=tan γ  (2)

In the above equation, r, t, ρ, and τ are the parameters, whose valuescan be determined from the best fit to the experimental results, and Sis the scalar order parameter. From equation (1), temperature dependenceof the alignment direction of the liquid crystal, which is determined byφ and θ, can be obtained. In other words, in the case of thetemperature-induced alignment transition, because of the couplingbetween φ and θ, the alignment direction of the liquid crystal movesalong the trajectory on the plane which is perpendicular to the YZ planeand which forms an angle γ of approximately 20° with the XY plane. Whenthe sample is observed with crossed nicols before being illuminated withlight, the optical anisotropic axis, that is, the alignment direction ofthe liquid crystal, is located at a position deviated by an angle φ ofapproximately 10° from the deposition plane YZ. It is understood thatwhen the temperature is increased, the optical anisotropic axis movesfrom the deposition plane toward the X axis. FIG. 2( a) schematicallyshows the directions of the optical anisotropic axes at differenttemperatures T₁ and T₂, and at a temperature near a clearing pointT_(N1) of 70° C., it is understood that the optical anisotropic axis isalong the X axis.

FIG. 3 shows the temperature-dependence of the azimuthal angle φaccording to the present invention, in which the vertical axisrepresents the azimuthal angle and the horizontal axis represents thetemperature. The results are very similar to those disclosed in theabove-mentioned literature (15). Before being illuminated with light,the temperature of the sample is set to 36° C. which is just below thetemperature at which the temperature-induced change of the opticalanisotropic axis occurs. For a very short period of time, such as onesecond or less, the sample is illuminated with excitation light whichpassed through a stop. It is found that optical anisotropic axis of apart of the sample illuminated with the light shifted to the X axis[FIG. 2( b)].

The sample illuminated with the light was placed at two differentpositions and was observed with crossed nicols, and the photographsthereof are shown in FIG. 4.

FIG. 4 are photographs of dichroic nematic liquid crystal which isplaced in the twofold degenerate anchoring condition by the SiO_(x) thinfilm, and this photograph was taken after the sample was illuminatedwith light having a wavelength λ of 510 to 560 nm for one second or lesswhile the temperature thereof was set to 36° C. The central region ofthe photograph is the illuminated position, and the two photographs showthe different places which were observed with crossed nicols. FIG. 4( a)shows the case in which the optical axis in the illuminated region islocated at the transmission position of one of the polarizers, and FIG.4( b) shows the case in which the sample was rotated in the clockwisedirection as indicated by the arrow so that the optical axis in theunilluminated region is located at the transmission position of thepolarizer.

As can be seen, the deviation of the optical axis is approximately 50°.A small alignment defect domain in the unilluminated region is used as amarker for recording the sample position.

The two different positions indicate the illuminated part in the celland the unilluminated part therein. From these two photographs, it isunderstood that the photoinduced rotation of the anisotropic axis in theplanar (in-plane) direction is approximately 50°. By a longer exposuretime of approximately two seconds, the change of the anisotropic axis isapproximately 80°.

In order to understand the origin of the photoinduced in-plane rotationof the optical anisotropic axis, it is necessary to recall thetemperature-induced anchoring transition of nematic liquid crystalcaused by the twofold degenerate anchoring [refer above-mentionedliteratures (15) and (18)] and the photoinduced anchoring transition ofphotodichroic nematic [refer above-mentioned literatures (4) to (6), and(9)].

According to the theoretical model of the temperature-induced alignmenttransition in the case of the twofold degeneracy [refer above-mentionedliterature (12)], the scalar order parameters at which the polar tiltangle θ vanishes and at which the azimuthal angle φ becomes 90° areS_(θ) and S_(φ), that is, S_(θ)=S(T_(θ)) and S_(φ)=S(T_(φ)) aresatisfied, respectively. The values obtained by fitting the experimentaldata to the theoretical model are such that S_(θ)=0.378 and S_(φ)=0.389are satisfied, that is, S_(θ)<S_(φ) is satisfied. Both S_(θ) and S_(φ)are larger than the scalar order parameter at an N-I phase transitionpoint [refer above-mentioned literature (12)].

The theoretical model predicts that two second-order transitions occurat T_(θ) and T_(φ), which are equal to or less than T_(NI). Accordingly,in the case of the twofold degenerate anchoring, by the coupling betweenθ and φ, when the sample temperature is increased, a quasi-in-plane(in-plane) reorientation of the alignment direction of the liquidcrystal will occur.

Next, dichroic nematic liquid crystal in a cell whose inner surfaces arecoated with SiO_(x) thin films will then be discussed (as has been wellknown, the surface of the SiO_(x) thin film is hydrophilic).

In general, during the trans-cis isomerization process, it is believedthat both the shape and the magnitude of a molecular dipole moment arechanged. In the case of the present invention, the molecular shape of4-hexyloxy-(4′-hexyl)azobenzene changes from linear to bent, as shown inFIG. 5. In addition, the magnitude of the molecular dipole momentincreases during the photoisomerization process from 0.5D (debye) forthe trans-isomer to 3.5D for the cis-isomer.

Through studies carried by the inventors of the present invention on theformation of LB (Langmuir-Blodgett) monolayers of azobenzene derivativeson a water surface, it has been understood that4-hexyloxy-(4′-hexyl)azobenzene acts as a molecule having polarity.Accordingly, due to the higher polarity, it is believed that, as shownin FIG. 6, the cis-isomer is very likely to be adsorbed on thehydrophilic SiO_(x) surface.

However, the presence of the cis-isomers will not only affect thesurface phenomenon but will also simultaneously influence the anchoringof the liquid crystal to the surface. Most probably, due to the presenceof the cis-isomers, the order parameters S_(θ) and S_(φ) of the liquidcrystal at the surface are reduced. Accordingly, the transitiontemperatures T_(θ) and T_(φ) are decreased. As a result, in the liquidcrystal cell in the twofold degenerate anchoring condition, in-planereorientation of the alignment occurs by illumination. That is, in otherwords, photoinduced reorientation of the alignment occurs at atemperature lower than that expected from the transition temperature ofthe temperature-induced anchoring transition.

To estimate the influence of the heating effect by excitation light, thesample temperature was set to a temperature close to the clearing pointT_(NI) and then the sample was illuminated with light for five seconds.A very small change (1° C. or less) in transition temperature T_(NI) wasobserved. From the result thus obtained, it is believed that thephotoinduced reorientation of the alignment caused by the heating effectof the excitation light is small enough to be ignored.

To control the alignment direction of the liquid crystal by light is ofgreat interest in view of photonics. In particular, fast in-planeswitching of the optical anisotropic axis of nematic liquid crystal bylight is very important in view of applications of all optical switches,optical modulators, and the like.

According to the present invention, it becomes clear that fast in-planeswitching can be performed by photoinduction of the optical anisotropicaxis of dichroic nematic liquid crystal. In particular, the change ofthe optical anisotropic axis by light is very large, such asapproximately 80°, and the speed is very fast, such as two seconds.However, according to design of dichroic liquid crystal, faster swichingmay be achived in future. Furthermore, by appropriately selecting thedepositon conditions for SiO_(x), it is believed that the photoinducedtransition could be 90°. As described above, in addition to liquidcrystal devices operated based on the amplitude modulation, large changein direction of the anisotropic axis is of great interest to devciesoperated based on the ligth phase modulation.

In addition, the present invention is not limited to the examplesdescribed above, and any modifications may be made without departingfrom the scope of the present invention and are not excluded therefrom.

As has thus been described, according to the present invention, fastin-plane switching can be performed by photoinduction of the opticalanisotropic axis of dichroic nematic liquid crystal. In particular, theswitching time is extremely short, such as approximately two seconds. Bythe design of dichroic liquid crystal, faster switching may be achievedin future. Furthermore, by selecting appropriate deposition conditionsfor SiO_(x), it is believed that the photoinduced phase transition couldbe 90°.

INDUSTRIAL APPLICABILITY

In addition to liquid crystal displays, the photoinduced switchingliquid crystal device according to the present invention can be appliedto the entire applied photo-techniques including optical communication,optical memories, and the like.

1. A photoinduced switching liquid crystal device comprising: a cellhaving substrates separated by a space and including a liquid crystal inthe space, the substrates having an alignment layer formed thereon,respectively, wherein the liquid crystal comprises a dichroic nematicliquid crystal having an azobenzene group, the alignment layer comprisesobliquely deposited silicon oxide, and a twofold degenerate in-planeswitching phenomenon, which is caused by competition of anchoring forceson the alignment layer, is combined with an anchoring transitionphenomenon, which is caused by a photoisomerization of liquid crystalmolecules having an azobenzene group, thereby realizing in-planeswitching of an optical anisotropic axis of the dichroic nematic liquidcrystal.
 2. The photoinduced switching liquid crystal device accordingto claim 1, wherein the diebroic nematic liquid crystal produces anapproximately 80 to 90° change of the optical anisotropic axis byphotoinduction.
 3. The photoinduced switching liquid crystal deviceaccording to claim 1, wherein the dichroic nematic liquid crystalproduces the in-plane switching on a time scale of seconds.
 4. Aphotoinduced switching liquid crystal device comprising: a firstsubstrate; a second substrate separated from the first substrate by aspace; and a liquid crystal provided in the space, wherein the liquidcrystal is dichroic nematic liquid crystal, the first and secondsubstrates have alignment layers comprising obliquely deposited siliconoxide, respectively, and the dichroic nematic liquid crystal comprises aphotoisomerizable material capable of transforming between first andsecond states having different electron dipole moments and producing aphotoinduced anchoring transition and a twofold degenerate in-planeswitching phenomenon, which is caused by competition of anchoring forceson the alignment layers, is combined with an anchoring transitionphenomenon, which is caused by photoisomerization of liquid crystalmolecules having an azobenzene group, thereby realizing in-planeswitching of an optical anisotropic axis of the photoisomerizablematerial.
 5. The photoinduced switching liquid crystal device accordingto claim 4, wherein the photoisomerizable material has an azobenzenegroup.
 6. The photoinduced switching liquid crystal device according toclaim 4, wherein the photoisomerizable material comprises4-hexyloxy-(4′-hexyl)azobenzene.